Methods and apparatus for producing semiconductor on insulator structures using directed exfoliation

ABSTRACT

Methods and apparatus provide for forming a semiconductor-on-insulator (SOI) structure, including subjecting a implantation surface of a donor semiconductor wafer to an ion implantation step to create a weakened slice in cross-section defining an exfoliation layer of the donor semiconductor wafer; and subjecting the donor semiconductor wafer to a spatial variation step, either before, during or after the ion implantation step, such that at least one parameter of the weakened slice varies spatially across the weakened slice in at least one of X- and Y-axial directions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional application claiming priority under 35 U.S.C. §120to U.S. patent application Ser. No. 12/290,362, which was allowed, filedon Oct. 30, 2008, now U.S Pat. No. 7,816,225, and entitled “METHODS ANDAPPARATUS FOR PRODUCING SOI STRUCTURES USING DIRECTED EXFOLIATION”, theentire disclosure of which is herby incorporated by reference.

TECHNICAL FIELD

The present invention relates to the manufacture ofsemiconductor-on-insulator (SOI) structures, such as those ofnon-circular cross section and/or those of relatively large crosssectional area.

BACKGROUND

Semiconductor on insulator devices are becoming more desirable as marketdemands continue to increase. SOI technology is becoming increasinglyimportant for high performance thin film transistors (TFTs), solarcells, and displays, such as, active matrix displays, organiclight-emitting diode (OLED) displays, liquid crystal displays (LCDs),integrated circuits, photovoltaic devices, etc. SOI structures mayinclude a thin layer of semiconductor material, such as silicon, on aninsulating material.

Various ways of obtaining SOI structures include epitaxial growth ofsilicon (Si) on lattice matched substrates, and bonding a single crystalsilicon wafer to another silicon wafer. Further methods includeion-implantation techniques in which either hydrogen or oxygen ions areimplanted either to form a buried oxide layer in the silicon wafertopped by Si in the case of oxygen ion implantation or to separate(exfoliate) a thin Si layer to bond to another Si wafer with an oxidelayer as in the case of hydrogen ion implantation.

U.S. Pat. No. 7,176,528 discloses a process that produces an SOG(semiconductor on glass) structure using an exfoliation technique. Thesteps include: (i) exposing a silicon wafer surface to hydrogen ionimplantation to create a bonding surface; (ii) bringing the bondingsurface of the wafer into contact with a glass substrate; (iii) applyingpressure, temperature and voltage to the wafer and the glass substrateto facilitate bonding therebetween; and (iv) separating the glasssubstrate and a thin layer of silicon from the silicon wafer.

The above approach is susceptible to an undesirable effect under somecircumstance and/or when employed in certain applications. Withreference to FIGS. 1A-1D, a semiconductor wafer 20 is implanted withions, e.g., hydrogen ions, through a surface 21, such that theimplantation dose is uniform in terms of density and depth across thesemiconductor wafer 20.

With reference to FIG. 1A, when a semiconductor material, such assilicon, is implanted with ions, such as H-ions, damage sites arecreated. The layer of damage sites define an exfoliation layer 22. Someof these damage sites nucleate into platelets with very high aspectratios (they have a very large effective diameter and almost no height).Gas resulting from the implanted ions, such as H₂, diffuses into theplatelets to form bubbles of comparably high aspect ratios. The gaspressure in these bubbles can be extremely high and has been estimatedto be as high as about 10 kbar.

As illustrated by the bi-directional arrows in FIG. 1B, the platelets orbubbles grow in effective diameter until they get close enough to eachother that the remaining silicon is too weak to resist the high pressureof the gas. As there is no preferential point for a separation front tostart, the multiple separating fronts are randomly created and themultiple cracks propagate through the semiconductor wafer 20.

Near the edges of the semiconductor wafer 20, a larger share ofimplanted hydrogen may escape from the hydrogen rich plane. This is sobecause of the proximity of sinks (i.e., the side walls of the wafer20). More particularly, during implantation, the ions (e.g., hydrogenprotons) decelerate through the lattice structure of the semiconductorwafer 20 (e.g., silicon) and displace some silicon atoms from theirlattice sites, creating the plane of defects. As the hydrogen ions losetheir kinetic energy, they become atomic hydrogen and define a further,atomic hydrogen plane. Both the defect plane and the atomic hydrogenplane are not stable in the silicon lattice at room temperature. Thus,the defects (vacancies) and the atomic hydrogen move toward one anotherand form thermally stable vacancy-hydrogen species. Multiple speciescollectively create a hydrogen rich plane. (Upon heating, the siliconlattice cleaves generally along the hydrogen rich plane.)

Not all vacancies and hydrogen undergo collapse into hydrogen-vacancyspecies. Some atomic hydrogen species diffuse away from the vacancyplane and eventually leave the silicon wafer 20. Thus, some of theatomic hydrogen does not contribute to cleavage of the exfoliation layer22. Near the edges of the silicon wafer 20, the hydrogen atoms have anadditional path to escape from the lattice. Therefore, the edge areas ofthe silicon wafer 20 may be lower in hydrogen concentration. The lowerconcentration of hydrogen results in the need for a higher temperatureor longer time to develop enough force to support separation.

Therefore, during the separation process, a tent-like structure 24 iscreated with edges that are not separated. At a critical pressure,fracture of the remaining semiconductor material occurs along relativelyweak planes, such as {111} planes (FIG. 1C) and the separation of theexfoliation layer 22 from the semiconductor wafer 20 is complete (FIG.1D). The edges 22A, 22B, however, are out of a major cleavage planedefined by the damage sites. This non-planar cleavage is not desirable.Other characteristics of the separation include that the exfoliatedlayer 22 can be described as having “mesas”, where the platelets orbubbles were, surrounded by “canyons”, where the fracture occurred. Itis noted that these mesas and canyons are not accurately shown in FIG.1D as such details are beyond the capabilities of reproduction at theillustrated scale.

Without limiting the invention to any theory of operation, the inventorsof the instant application believe that the time from the onset ofseparation to completed separation is on the order of 10's ofmicro-seconds using the techniques described above. In other words, therandom onset and propagation of the separation is on the order of about3000 meters/sec. Again, without limiting the invention to any theory ofoperation, the inventors of the instant application believe that thisrate of separation contributes to the undesirable characteristic of thecleaved surface of the exfoliation layer 22 described above (FIG. 1D).

U.S. Pat. No. 6,010,579 describes a technique of uniform ionimplantation into a semiconductor substrate 10 to a uniform depth Z0,taking the wafer to a temperature below that which would initiate theonset of separation, and then introducing multiple impulses of energy tothe edge of the substrate 10 in the vicinity of the implant depth Z0 inorder to achieve a “controlled cleave front”. U.S. Pat. No. 6,010,579states that the above approach is an improvement over so-called “random”cleavage at least as to surface roughness. The instant invention takes adirected separation approach that is significantly different from the“controlled cleave front” approach of U.S. Pat. No. 6,010,579 anddifferent from the “random” cleaving approach.

The challenges associated with the separation of the exfoliation layer22 from the semiconductor wafer 20 discussed above are exacerbated asthe size of the SOI structure increases, and particularly when the shapeof the semiconductor wafer is rectangular. Such rectangularsemiconductor wafers may be used in applications where multiplesemiconductor tiles are coupled to an insulator substrate. Furtherdetails regarding the manufacturing of a tiled SOI structure may befound in U.S. Application Publication No. 2007/0117354, the entiredisclosure of which is hereby incorporated by reference.

SUMMARY

For ease of presentation, the following discussion will at times be interms of SOI structures. The references to this particular type of SOIstructure are made to facilitate the explanation of the invention andare not intended to, and should not be interpreted as, limiting theinvention's scope in any way. The SOI abbreviation is used herein torefer to semiconductor-on-insulator structures in general, including,but not limited to, silicon-on-insulator structures. Similarly, the SOGabbreviation is used to refer to semiconductor-on-glass structures ingeneral, including, but not limited to, silicon-on-glass structures. Theabbreviation SOI encompasses SOG structures.

In accordance with one or more embodiments of the present invention,method and apparatus employed in forming a semiconductor-on-insulator(SOI) structure, provide for: subjecting a implantation surface of adonor semiconductor wafer to an ion implantation step to create aweakened slice in cross-section defining an exfoliation layer of thedonor semiconductor wafer; and subjecting the donor semiconductor waferto a spatial variation step, either before, during or after the ionimplantation step, such that one or more parameters of the weakenedslice vary spatially across the wafer in at least one of X- and Y-axialdirections.

The spatial variation step facilitates characteristics of separation ofthe exfoliation layer from the donor semiconductor wafer such that suchseparation is directionally and/or temporally controllable.

The parameters may include one or more of the following, alone or incombination: (i) densities of nucleation sites resulting from the ionimplantation step; (ii) depths of the weakened slice from theimplantation surface (or the reference plane); (iii) artificiallycreated damage locations (e.g., blind holes) through the implantationsurface at least to the weakened slice; and (iv) nucleation of defectsites and/or pressure increases throughout the weakened slice usingtemperature gradients.

The method and apparatus further provide for elevating the donorsemiconductor wafer to a temperature sufficient to initiate separationat the weakened slice from a point, edge, and/or region of the weakenedslice. The donor semiconductor wafer may be subject to furthertemperatures sufficient to continue separation substantially along theweakened slice directionally as a function of the varying parameter(s).

Other aspects, features, advantages, etc. will become apparent to oneskilled in the art when the description of the invention herein is takenin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purposes of illustrating the various aspects of the invention,there are shown in the drawings forms that are presently preferred, itbeing understood, however, that the invention is not limited to theprecise arrangements and instrumentalities shown.

FIGS. 1A, 1B, 1C, and 1D are block diagrams illustrating an exfoliationprocess in accordance with the prior art;

FIGS. 2A-2B are block diagrams illustrating an exfoliation process inaccordance with one or more aspects of the present invention;

FIG. 3A is a top view of a donor semiconductor wafer having a spatiallyvarying parameter associated with a weakened layer or slice therein inaccordance with one or more aspects of the present invention;

FIG. 3B is a plot that graphically illustrates the spatially varyingparameter of FIG. 3A;

FIG. 3C is a plot that graphically illustrates that the spatiallyvarying parameter of FIG. 3A is the depth of the weakened slice;

FIGS. 4A, 4B, and 4C are top views of respective donor semiconductorwafers having further spatially varying parameters in accordance withone or more further aspects of the present invention;

FIGS. 5A, 5B, and 5C are simplified diagrams of some ion implantationapparatus that may be adapted to achieve spatially varying parameters ofthe donor semiconductor wafer;

FIGS. 6A-6B illustrate an ion implantation technique that may be adaptedto achieve a spatially varying density of nucleation sites in the donorsemiconductor wafer;

FIGS. 7A-7B illustrate an ion implantation technique that may be adaptedto achieve a spatially varying implantation depth in the donorsemiconductor wafer;

FIGS. 7C-7D are graphs illustrating relationships between tilt angle ofion implant and implant depth;

FIGS. 8A-8B illustrate an ion implantation technique that may be adaptedto achieve a spatially varying ion implantation distribution width inthe donor semiconductor wafer;

FIG. 8C is a graph illustrating a relationship between ion implant tiltangle and straggle;

FIGS. 9A-9D illustrate a further ion implantation technique that may beadapted to achieve a spatially varying ion implantation depth in thedonor semiconductor wafer;

FIGS. 10A-10D and 11 illustrate a further ion implantation techniquethat may be adapted to achieve a spatially varying distribution ofdefect sites in the donor semiconductor wafer; and

FIGS. 12A-12B illustrate a time-temperature profile technique that maybe adapted to achieve a spatially varying parameter profile in the donorsemiconductor wafer.

DETAILED DESCRIPTION

With reference to the drawings, wherein like numerals indicate likeelements, there is shown in FIGS. 2A-2B an intermediate SOI structure(in particular, an SOG structure) in accordance with one or moreembodiments of the present invention. The intermediate SOG structureincludes an insulator substrate, such as a glass or glass ceramicsubstrate 102, and a donor semiconductor wafer 120. The glass or glassceramic substrate 102 and the donor semiconductor wafer 120 have beencoupled together using any of the art-recognized processes, such asbonding, fusion, adhesive, etc.

Prior to coupling the glass or glass ceramic substrate 102 and the donorsemiconductor wafer 120 together, the donor semiconductor wafer 120includes an exposed implantation surface 121. The implantation surface121 of the donor semiconductor wafer 120 is subjected to an ionimplantation step to create a weakened slice 125 in cross-sectiondefining an exfoliation layer 122. The weakened slice 125 liessubstantially parallel to a reference plane (which could be anywhere,and thus is not illustrated) defined by X-Y orthogonal axial directions.The X-axial direction is shown left-to-right in FIG. 2A, while theY-axial direction is orthogonal to the X-axial direction into the page(and thus is not shown).

The donor semiconductor wafer 120 is subject to a spatial variationstep, either before, during or after the ion implantation step, suchthat the characteristics of separation of the exfoliation layer 122 fromthe donor semiconductor wafer 120 are directionally and/or temporallycontrollable. While not intending to limit the invention to any theoryof operation, it is believed that such directional and/or temporalcontrollability may result in improved separation characteristics, suchas smoother exposed surfaces on the exfoliation layer 122 and the donorsemiconductor wafer 120 (post separation). It is also believed that suchdirectional and/or temporal controllability may result in improved edgecharacteristics, e.g., improving the yield of edges of the exposedsurfaces on the exfoliation layer 122 and the donor semiconductor wafer120 that are in a major cleavage plane defined by the weakened slice125.

The directionally and/or temporally controllable characteristics ofseparation of the exfoliation layer 122 from the donor semiconductorwafer 120 may be achieved in a number of ways, such as by varying one ormore parameters spatially across the weakened slice 125 in at least oneof the X- and Y-axial directions. The parameters may include one or moreof the following, alone or in combination: (i) densities of nucleationsites resulting from the ion implantation step; (ii) depths of theweakened slice 125 from the implantation surface 121 (or the referenceplane); (iii) artificially created damage locations (e.g., blind holes)through the implantation surface 121 at least to the weakened slice 125;and (iv) nucleation of defect sites and/or pressure increases throughoutthe weakened slice 125 using temperature gradients.

As illustrated in FIGS. 2A-2B by the arrow A, the directionally and/ortemporally controllable characteristics of separation of the exfoliationlayer 122 from the donor semiconductor wafer 120 result in a propagatingseparation from one point, edge, and/or region to other points, edges,and/or regions of the weakened slice 125 as a function of time. This isgenerally achieved as follows: first, varying the one or more parametersspatially across the weakened slice 125 as discussed above, and second,elevating the donor semiconductor wafer 120 to a temperature sufficientto initiate separation at the weakened slice 125 from such point, edge,and/or region. Thereafter, the donor semiconductor wafer 120 is elevatedto further temperatures sufficient to continue separation substantiallyalong the weakened slice 125 directionally as a function of the spatialvariation of the parameter(s) across the weakened slice 125. The varyingparameter is preferably established such that a time-temperature profileof the elevating temperatures is on the order of seconds, and apropagation of the separation along the weakened slice 125 occurs overat least one second.

Reference is now made to FIGS. 3A-3C, which illustrate further detailsassociated with varying the one or more parameters spatially across theweakened slice 125. FIG. 3A is a top view of the donor semiconductorwafer 120 viewed through the implantation surface 121. The variation inshading in the X-axial direction represents the spatial variation in theparameter (e.g., density of nucleation sites, pressure within the sites,degree of nucleation, distribution of artificially created damage sites(holes), implantation depth, etc.). In the illustrated example, the oneor more parameters vary from one edge 130A in the X-axial directiontoward an opposite edge 130B of the donor semiconductor wafer 120 (andthus the weakened slice 125 thereof), or vice verse.

With reference to FIG. 3B, a graph of the separation parameterillustrates the cross-sectional profile of, for example, the density ofnucleation sites within the weakened slice 125 as a function of theX-axial direction. Alternatively, or in addition, the separationparameter may represent one or more of the pressure within thenucleation sites, the degree of nucleation, the distribution ofartificially created damage sites (holes), etc., each as a function ofthe X-axial, spatial metric. With reference to FIG. 3C and thecross-sectional reference 3C-3C in FIG. 3A, a graph of the separationparameter is illustrates with respect to the cross-sectional profile of,for example, the depth of the weakened slice 125 (e.g., corresponding tothe ion implantation depth) as a function of the X-axial direction.

While not intending to limit the invention to any theory or theories ofoperation, it is believed that the propagation of separation(illustrated by broken arrows) from the edge 130A toward the edge 130Boccurs when the density of nucleation sites is relatively high at edge130A and reduces to lower densities of nucleation sites at spatiallocations toward the edge 130B. This theory is also believed to hold inconnection with other parameters, such as the gas pressure within thenucleation sites, the degree of merging nucleation sites prior toseparation, and the distribution of artificially created damage sites(holes). As to the parameter associated with the depth of the weakenedslice 125, however, it is believed that the propagation of separation(illustrated by solid arrows) from the edge 130B toward the edge 130Aoccurs when a substantially low depth exists along the initial edge 130Bof the weakened slice 125 and comparatively higher depths exist atsuccessively further distances toward the edge 130A.

Reference is now made to FIGS. 4A-4C, which illustrate further detailsassociated with varying the one or more parameters spatially across theweakened slice 125. The figures show top views of the donorsemiconductor wafer 120 viewed through the implantation surface 121. Thevariation in shading in the X- and Y-axial directions represent thespatial variation in the parameter, again density of nucleation sites,pressure within the sites, degree of nucleation, distribution ofartificially created damage sites (holes), implantation depth, etc. Ineach illustrated case, the parameter is varied spatially in both the X-and Y-axial directions.

With specific reference to FIG. 4A, the shading may represent that theparameter varies spatially starting from two edges 130A, 130D towardother edges 130B, 130C and varying at successively further distances inboth the X- and Y-axial directions. In keeping with the discussionabove, when considering the parameter of the density of nucleationsites, if the higher densities initiate at edges 130A, 130D, then it isbelieved that the propagation of separation (illustrated by the brokenarrow) will radiate out from the corner of edges 130A, 130D toward thecenter of the wafer 120 and toward the other edges 130B, 130C. Thistheory is also believed to hold in connection with other parameters,such as the gas pressure within the nucleation sites, the degree ofmerging of nucleation sites prior to separation, and the distribution ofartificially created damage sites (holes). As to the parameterassociated with the depth of the weakened slice 125, however, it isbelieved that the propagation of separation (illustrated by the solidarrow) will radiate out from the corner of edges 130B, 130C toward thecenter of the wafer 120 and toward the other edges 130A, 130D when thelower depths low depth initiate along the edge 130B, 130C.

With specific reference to FIGS. 4B and 4C, the shading may representthat the parameter varies spatially starting from all edges 130 andvarying toward the center of the donor semiconductor wafer 120, or viceverse.

Further details will now be provided with reference to the particularparameter of spatially varying the densities of nucleation sitesresulting from ion implantation across the weakened slice 125 in one orboth of the X- and Y-axial directions. No matter what technique isemployed to achieve such spatial variation, it is preferred that amaximum density of nucleation sites exists at one or more edges, points,or regions of the weakened slice 125 of about 5×10⁵ sites/cm² and aminimum density of nucleation sites exists spaced away therefrom in theweakened slice 125 of about 5×10⁴ sites/cm². Looking at the variation inanother way, a difference between the maximum density of nucleationsites and the minimum density of nucleation sites may be between about10 fold.

In accordance with one or more aspects of the present invention, thedensity of nucleation sites within the weakened slice 125 may be variedspatially by varying the dose of the ion implantation step. By way ofbackground, the weakened slice 125 (and thus the exfoliation layer 122)is created by subjecting the implantation surface 121 to one or more ionimplantation steps. Although there are numerous ion implantationtechniques, machines, etc. that may be utilized in this regard, onesuitable method dictates that the implantation surface 121 of the donorsemiconductor wafer 120 may be subject to a hydrogen ion implantationstep to at least initiate the creation of the exfoliation layer 122 inthe donor semiconductor wafer 120.

With reference to FIG. 5A, a simplified schematic of an Axcelis NV-10type batch implanter is illustrated, which may be modified for use inspatially varying the density of nucleation sites within the weakenedslice 125 by varying the dose of implanted ions.

Multiple donor semiconductor wafers 120, in this case rectangular tiles,may be distributed azimuthally at a fixed radius on a platen 200relative to the incident ion beam 202 (directed into the page). Rotationof the platen 200 provides a pseudo-X-scan (dX/dt) while mechanicaltranslation of the entire platen 200 provides the Y-scan (dY/dt). Theterm pseudo-X-scan is used because for small radius platens 200, theX-scan is somewhat more curved as compared to larger radius platens 200,and thus, perfectly straight scans are not obtained on such rotatingplatens 200. Modulating the X-scan speed and/or the Y-scan speed willresult in spatial variation in the dose. Increasing the Y-scan speed asthe ion beam 202 travels radially toward the center of the platen 200has been used in the past to ensure a uniform dose. Indeed, as theconventional thinking in the art is to achieve a spatially uniform dose,and as the angular speed relative to the donor semiconductor wafers 120decreases closer to the center of the platen 200, the Y-scan speed mustcorrespondingly increase. In accordance with the invention, however, aspatially varying dose may be achieved by not adhering to theconventional scan protocol, resulting in the patterns of, for example,FIGS. 3A and 4A. For example, leaving the Y-scan speed uniform as theion beam 202 travels radially toward the center of the platen 200.Alternatively, one could decrease the Y-scan speed as the ion beam 202travels radially toward the center of the platen 200. Those skilled inthe art will recognize other possibilities from the disclosure herein.An alternative approach is to vary the beam energy as a function of thescan rates and positions. These changes may be effected throughmodification to the control algorithm of the implanter in software, anelectronic interface between the controlling software and the endstation drive, or other mechanical modification.

With reference to FIG. 5B, a simplified schematic of a single-substrateX-Y implanter is illustrated, which also may be modified for use inspatially varying the density of nucleation sites within the weakenedslice 125 by varying the dose of implanted ions. In this case, theelectronic beam 202 is scanned much faster than the mechanical substratescan (of FIG. 5A). Again, the conventional thinking in the art is toachieve a spatially uniform dose, and thus the X and Y scanning ratesand beam energy are set such that the uniform dose is achieved. Again,spatially varying dosages may be achieved by not adhering to theconventional scanning protocol. Significant spatial variation in implantdosage may be achieved through numerous combinations of variable X and Yscanning rates and/or beam energy. One-dimensional or two-dimensionalgradients may be produced, either vertical or horizontal, through suchvariation resulting in the patterns of, for example, FIGS. 3A, 4A, 4Band 4C.

With reference to FIG. 5C, a simplified schematic of an implanter isillustrated in accordance with ion shower techniques. A ribbon beam 204arises from an extended ion source. In accordance with conventionaltechniques, a single uniform speed scan (in proportion with a uniformbeam energy in the orthogonal direction) can achieve conventionalideals, i.e., a spatially uniform dose. In accordance with variousaspects of the invention, however, a one-dimensional gradient (e.g.,that of FIG. 3A rotated 90 degrees) may be produced through variation inthe mechanical scanning rate of the donor semiconductor wafers 120through the ribbon beam 204. Twisting the donor semiconductor wafers 120by some angle relative to the ribbon beam 204 in combination withvariation in the mechanical scanning rate may produce spatial variationin the dose in a manner similar to that of FIG. 4A. Alternatively oradditionally, a spatially varying beam current along the beam sourcewould provide an orthogonal gradient to the scan direction, providingadditional degrees of freedom to produce the subject spatially varyingdosages.

Irrespective of the particular implantation technique employed toachieve the variation in dose, and irrespective of the location of thehighest dose (e.g., along one or more initial edges, an initial point,or an initial region), the substantially highest dose is within somedesirable range in units of atoms/cm² and the lowest dose furthertherefrom in at least one of the X- and Y-axial directions is withinsome other desirable range in units of atoms/cm². A difference betweenthe maximum dose and the minimum dose may be between about 10-30%, witha maximum variation of about a factor of three. In some applications, adifference of at least about 20% has been found to be important.

In accordance with one or more further aspects of the present invention,the density of nucleation sites within the weakened slice 125 may bevaried spatially by implanting a first species of ions in asubstantially uniform manner to establish the weakened slice 125 with asubstantially uniform distribution. Thereafter, the donor semiconductorwafer 120 may be implanted with a second species of ions in asubstantially non-uniform manner. The non-uniform implantation isestablished such that the second species of ions causes migration ofatoms to the weakened slice 125 resulting in the spatially varyingdensities of nucleation sites across the weakened slice 125.

By way of example, the first species of ions may be hydrogen ions andthe second species of ions may be helium ions.

The non-uniform implantation may take place using the techniquesdescribed above, described later in this description, or gleaned fromother sources. For example, the dose of the second species of ions maybe spatially varied. The variation in the dose of the second species ofions (such as He ions) will cause a subsequent non-uniform migration ofthe second species to the location of the first species, therebyestablishing a non-uniform density of nucleation sites. This variationwill probably also vary the pressure in the platelets, which could alsobe beneficial.

Alternatively, the non-uniform implantation of the second species ofions may include implanting the second species of ions to varying depthsspatially across the donor semiconductor wafer 120. Any of the knowntechniques for implanting ions to uniform depths may be modified bythose skilled in the art in accordance with the teaching herein toachieve non-uniform depth profiles. By way of background, it is knownthat He ions can be implanted deeper than H, for example, as much as twotimes deeper or more. As the wafer temperature increases, much of the Heions will migrate to the site of shallower H ion implants and willprovide the gas pressure for later separation. In accordance with theinstant aspect of the invention, the damage caused by more deeply buriedHe is located at a depth in the donor semiconductor wafer 120 far fromthe shallower H ion implant and fewer of such He ions will arrive therein a given time. The opposite is true for the less deeply implanted Heions, thereby resulting in a spatially varying density of nucleationsites across the weakened slice 125.

While, theoretically, the spatially varied density of nucleation sitesmay be achieved irrespective of the order of the first and secondspecies of ions (e.g., He implanted first or H implanted first), theorder of the multiple ion implantation steps may also contribute to thedesired result. Indeed, the order of implantation, depending on ionspecies, may have an overall effect on the density even as the densityalso varies spatially. While counterintuitive and surprising to manyskilled artisans, it has been found that H implanted first creates morenucleation sites. For a given dose, He is recognized by skilled artisansto produce about ten times the damage as H ions. It should be noted,however, that the damage produced by the He ions (a vacancy andinterstitial semiconductor atom, or Frankel pair) self anneals rapidlyeven at room temperature. Thus, much, but not all, of the He damage isrepaired. H ions, on the other hand, bond with semiconductor atoms, suchas Si atoms (forming an Si—H bond), and stabilize the damage that iscreated. If H is present before the He is implanted, more nucleationsites are created.

Reference is now made to FIGS. 6A-6B, where a further example isillustrated that may be suitable for achieving the spatial variation inthe density of nucleation sites. In this example, as illustrated in FIG.6A, the spatial variation in the density of nucleation sites is achievedby adjusting the beam angle of the ion beam during the ion implantationstep. Although the beam angle may be adjusted in a number of ways, onesuch approach is to tilt the donor semiconductor wafer 120 with respectto the ion beam (e.g., a dot beam 202) as illustrated in FIG. 6A. Thedonor semiconductor wafer 120 has a width (left-to-right as shown on thepage), a depth (into the page) and a height (top-to-bottom as shown onthe page). The width and depth may define the X- and Y-axial directions,and the height may define a longitudinal axis, Lo, normal to theimplantation surface 121. The donor semiconductor wafer 120 is tiltedsuch that the longitudinal axis Lo thereof is at an angle Φ with respectto a directional axis of an ion implantation beam (shown as a solidarrow) during the ion implantation step. The angle Φ may be betweenabout 1 to 45 degrees.

Under a tilt condition, as the beam source scans from location A tolocation B, the width of the beam 202 varies at the implantation surface121 of the donor semiconductor wafer 120 from a width Wa to a width Wb,or vice verse. The variation in width contributes to a variation in thedensities of nucleation sites resulting from the ion implantation in thescanning directions (which may be set up to vary along at least one ofthe X- and Y-axial directions).

The implant beam 202 may include hydrogen ions, which have the same(positive) electrical charge. As particles with the same charge repeleach other, the beam 202 is wider at a longer distance from ion source(position A), and narrower at a shorter distance from ion source(position B). The more focused (lower width Wb) ion beam at position Bheats the local area of the donor semiconductor wafer 120 to a higherdegree than the less focused (higher width Wa) ion beam at position A.Under higher temperature, more hydrogen ions diffuse out from such localarea, and a lower share of hydrogen ions remain as compared to otherareas. As illustrated in FIG. 6B, this results in laterally non-uniformdistribution of hydrogen (and thus the density of nucleation sites) inthe weakened slice 125 of the donor semiconductor wafer 120.

Similar spatial variation in the density of nucleation sites may beachieved by adjusting the angle of the beam source or incorporating someof the known mechanisms for adjusting the collimation of the ion beam202.

A further technique that may be suitable for achieving the spatialvariation in the density of nucleation sites is to employ a two-stageion implantation step. A first ion implantation is performed to implantions that have the effect of attracting a second species of ions.Thereafter, the second species of ions are implanted. The first speciesof ions are implanted in a spatially non-uniform manner, using any ofthe suitable techniques described above or later herein. Thus, when thesecond species of ions are implanted, and migrate to the first species,the resultant weakened slice 125 exhibits a non-uniform density ofnucleation sites.

For example, the first ion species may be based on the material of thedonor semiconductor wafer 120, such as using silicon ions forimplantation in a silicon donor semiconductor wafer 120. Such Si ionsmay have the property of trapping a second species of ions, such ashydrogen ions. As noted above, H ions bond with some semiconductoratoms, such as Si atoms, forming an Si—H bond. As an example,silicon-into-silicon implantation may be performed at doses and energiesknown in the art, such as is described in U.S. Pat. No. 7,148,124, theentire disclosure of which is incorporated by reference. Unlike theprior art, however, a spatial density distribution of the trapping ionspecie (in this case Si) is non-uniform (e.g., highest at one edge andlowest on an opposite edge of the donor semiconductor wafer 120, orother variations discussed herein). Next, a second species of ions, suchas hydrogen, is implanted, which may be a uniform distribution. Theamount of hydrogen remaining in the weakened slice 125 of the donorsemiconductor wafer 120 will depend on two factors: (1) theconcentrating distribution of sites that are able to trap the secondspecies, hydrogen, and (2) the available hydrogen (the hydrogenimplanted and remaining from the implant dose).

It is noted that the non-uniform spatial distribution of the species maybe reversed to achieve a similar result. For example, the first speciesmay implanted uniformly, followed by a non-uniform implantation of thesecond species. Alternatively, both implants may be spatiallynon-uniform. The non-uniform distribution of the second species (e.g.,hydrogen) within the weakened slice 125 results in a point, edge orregion of highest concentration of hydrogen, which in turn is locationof the lowest temperature for initiating cleavage.

Again, with reference to FIGS. 2A-2B, the arrow A illustrates thedirectionally and/or temporally controllable characteristics ofseparation of the exfoliation layer 122 from the donor semiconductorwafer 120, where a propagating separation from one a point, edge, and/orregion to other points, edges, and/or regions of the weakened slice 125is achieved as a function of time. In the context of spatial variationof the density of nucleation sites, the donor semiconductor wafer 120 iselevated to a temperature sufficient to initiate separation at theweakened slice 125 from a point, edge, and/or region of highest density.It has been found that high hydrogen concentrations in silicon allowsseparation at temperatures as low as 350° C. or lower, while siliconwith lower concentrations of hydrogen separates at higher temperatures,such as 450° C. or more. The donor semiconductor wafer 120 is elevatedto further temperatures sufficient to continue separation substantiallyalong the weakened slice 125 directionally as a function of the spatialvariation of the density across the weakened slice 125.

Further details will now be provided with reference to the particularparameter of spatially varying the depth of the weakened slice 125resulting from ion implantation in one or both of the X- and Y-axialdirections. No matter what technique is employed to achieve such spatialvariation, it is preferred that a substantially low depth is betweenabout 200-380 nm and a highest depth is between about 400-425 nm.Looking at the variation in another way, a difference between themaximum and minimum depths may be between about 5-200%.

In accordance with one or more aspects of the present invention, thedepth of the weakened slice 125 may be varied spatially by adjustingbeam angle of the ion beam during the ion implantation step. Indeed, theprocess discussed with respect to FIGS. 6A-6B may have applicability toadjusting the depth of the weakened slice 125. (It is noted that themechanism of varying temperature as a function of beam width is notbelieved to be the reason that variations in the depth of the weakenedslice 125 are achieved.)

With reference to FIGS. 6A, and 7A-7B, the spatial variation in thedepth of the weakened slice 125 may be achieved by varying at least oneof: (1) the angle Φ of tilt (shown and described with reference to FIG.6A); and (2) a twist of the donor semiconductor wafer 120 about thelongitudinal axis Lo thereof with respect to the directional axis of theion implantation beam 202. Adjustments in the tilt and/or twist are madeto adjust a degree of channeling through the lattice structure of thedonor semiconductor wafer 120, where such channels tend to align andmisalign with the ion beam 202 as the ion beam 202 scans across theimplantation surface 121. As the degree of channeling varies spatially,so does the depth of the weakened slice 125.

The angle Φ may be between about 1-10 deg degrees and the angle of twistmay be between about 1-45 degrees.

As inferred above, and with further reference to FIGS. 7C and 7D,implant depth gets smaller as tilt gets bigger. For relatively smallangles (e.g., 0-10 deg), the relationship between implant depth and tiltis dominated by channeling. For relatively larger angles, the cosineeffect dominates. In other words, the resultant exfoliation filmthickness is essentially proportional to the cosine of the implantangle.

Alternatively or additionally, the spatial variation step may includevarying an energy level of the ion beam 202 such that as the ion beam202 scans across the implantation surface 121 of the donor semiconductorwafer 120, depths of the weakened slice 125 from the implantationsurface 121 vary spatially across the donor semiconductor wafer 120.

As illustrated in FIG. 7B, the above techniques results in a laterallynon-uniform depth of the weakened slice (or implant depth) of the donorsemiconductor wafer 120.

In connection with adjusting the tilt of the donor semiconductor wafer202, a further parameter that may be exploited to achieve spatialvariations is the width of the ion deposition distribution (orstraggle). As illustrated in FIG. 8A, the width of the ion distributionthrough the weakened slice 125 (top-to-bottom) varies as a function ofthe angle of the tilt of the donor semiconductor wafer 120 (or moregenerally the beam angle). Thus, by varying the tilt angle, a spatiallyvarying distribution width may be achieved in the weakened slice 125 (asillustrated in FIG. 8B). While not intending to be limited by any theoryof operation, it is believed that the portions of the weakened slice 125having narrower distribution widths will separate at lower temperaturesthan the portions of the weakened slice 125 having wider distributionwidths. Thus, it is believed that directionally and/or temporallycontrollable characteristics of separation of the exfoliation layer 122from the donor semiconductor wafer 120, where a propagating separationfrom one a point, edge, and/or region to other points, edges, and/orregions of the weakened slice 125 may be achieved as a function of timeand temperature.

With reference to FIG. 8C, additional data regarding the effect of thetilt on the straggle, which again has an impact on the width of theimplant profile. The dose used in both implants illustrated in FIG. 8Care the same. Although the peak H concentration is different, bothimplants exfoliate. Thus, the difference between a tilt variation of+/−0.1 deg and +/−3 deg is significant for straggle.

With reference to FIGS. 9A-9D, another technique for spatially varyingthe depth of the weakened slice 125 includes subjecting the donorsemiconductor wafer 120 to a post implantation material removal processsuch that the depths of the weakened slice 125 from the implantationsurface 121 vary spatially across the donor semiconductor wafer 120. Asillustrated in FIG. 9A, the donor semiconductor wafer 120 may be subjectto some deterministic polishing process or plasma-assisted chemicaletching (PACE). These techniques permit local control of the amount ofmaterial removed by the polishing step. Other methods, includingReactive Ion Etching (RIE), Chemical Mechanical Polishing (CMP), and wetchemical etching may also have non-uniform material removal across theexposed surface which is regular and reproducible. One or more of theseor other techniques may be used to introduce slight variation in thedepth of the weakened slice 125 from the implantation surface 121, suchas any of those illustrated in FIGS. 3A, 4A, 4B, 4C, and others. The ionimplantation step prior to material removal may be spatially uniform ornon-uniform.

With reference to FIGS. 9B and 9C, the spatial variation step mayinclude using a mask 220A or 220B on the implantation surface 121 of thedonor semiconductor wafer 120 in a spatially non-uniform manner suchthat penetration of the ions is impeded to varying degrees as the ionbeam 202 scans across the implantation surface 121. The masking film 220may include silicon dioxide, organic polymers such as photoresist, andothers. Possible deposition techniques include plasma-enhanced chemicalvapor deposition (PECVD), spin coating, Polydimethylsiloxane (PDMS)stamping, etc. The masking film 220 thickness may be less than orcomparable to the intended depth of the weakened slice 125. As the depthto which ions are implanted is determined by the energy of the incidentions, the impeding action of the mask 220 will translate into spatialmodulation in primarily the depth of the implanted species in the donorsemiconductor wafer 120. Depending on the characteristics of thedeposited mask 220, the desired characteristic may be achieved by addinglength to the ion path, scattering the ions to alter the degree ofchanneling, or other phenomena.

As illustrated in FIG. 9D (which illustrates lower depths on all edgesof the weakened slice 125 and higher depths toward the center thereof),after or during bonding to the substrate 102, the donor semiconductorwafer 120 is elevated to a temperature sufficient to initiate separationat the weakened slice 125 from a point, edge, and/or region of lowestdepth. The donor semiconductor wafer 120 is elevated to furthertemperatures sufficient to continue separation substantially along theweakened slice 125 directionally as a function of the spatial variationof the depth from lowest depth to highest depth.

With reference to FIGS. 10A-10D and 11, the spatial variation step mayinclude boring one or more blind holes 230 through the implantationsurface 121 at least to the weakened slice 125, and preferably throughthe weakened slice 125 (FIG. 10B). While not intending to limit theinvention to any theory of operation, it is believed that during orafter bonding to the substrate 102 (FIG. 10C), elevating the donorsemiconductor wafer 120 to higher temperature will initiate separationat the blind hole 230 (FIG. 10D) prior to separation at locationswithout such hole. As illustrated in FIG. 11, boring an array of blindholes 230 through the implantation surface 121 may create a non-uniformspatial distribution of such holes. Thus, elevating the donorsemiconductor wafer 120 to temperatures sufficient to initiate andcontinue separation substantially along the weakened slice 125 may beachieved directionally as a function of the distribution of the array ofblind holes 230, from highest to lowest concentration.

With reference to FIGS. 12A-12B, the spatial variation step may includesubjecting the donor semiconductor wafer 120 to a non-uniformtime-temperature profile such that the nucleation site density orpressure at respective spatial locations throughout the weakened slice125 vary spatially across the donor semiconductor wafer 120. Forexample, the illustrated temperature gradient in FIG. 12A applies ahigher temperature to the left side of the wafer 120 as compared to theright side. This temperature gradient may be applied either beforebonding or in-situ during bonding to the substrate 102. Over time, ifthe process time is kept below the separation threshold for the givenprocess temperature, at least one of the nucleation of defect sites andthe gas pressure therein increases throughout the weakened slice 125 invarying degrees, spatially across the wafer 120 as a function of thetemperature gradient (see FIG. 12B). The separation threshold time for agiven process temperature is expected to follow an Arrheniusrelationship, where the separation threshold time is exponentiallyproportional to the inverse of the process temperature. The parameter ofinterest is the ratio of the process time to the separation thresholdtime at the process temperature. Any of the aforementioned spatiallyvarying parameter profiles discussed herein or otherwise desirable maybe achieved by adjusting the process time-separation time ratio profile.Then, the donor semiconductor wafer 120 is elevated to a temperaturesufficient to initiate separation at the weakened slice 125 from apoint, edge, and/or region of maximum process time-separation timeratio. In the illustrated example, the maximum process time-separationtime ratio is on the left side of the wafer 120. The donor semiconductorwafer 120 is then elevated to further temperatures sufficient tocontinue separation substantially along the weakened slice 125directionally as a function of the varying time-temperature profile,from maximum process time-separation time ratio(s) to minimum processtime-separation time ratio(s). Depending on material characteristics andother factors, including ion species, dose, and implant depth, thesubstantially high process time-separation time ratio is between about0.9 and 0.5 and a lowest process time-separation time ratio is betweenabout 0 and 0.5.

Various mechanisms may be used pre-bonding or in-situ bonding to achievethe spatially varying time-temperature profile. For example, one or morespatially non-uniform conductive, convective, or radiating heatingtechniques (hotplate, laser irradiation, visible/infrared lamp, orother) may be employed to heat the donor semiconductor wafer 120.Controlled time/temperature gradients may be achieved by direct orindirect thermal contact (conduction) to achieve any of the desirableprofiles. An addressable, two-dimensional array of hotplate elements maybe used to achieve different profiles based on computer control orprogramming. Localized infrared radiation, employing, for example, alamp as used in rapid thermal annealing (radiation) may be employed,and/or visible or near-infrared laser radiation may be used to providelocalized and spatially non-uniform heating (radiation). Alternatively,application of a uniform or non-uniform thermal profile through anymeans and application of a spatially non-uniform cooling mechanism, suchas direct contact (conductive), or gas or fluid flow jets(conductive/convective), may be employed to achieve the desiredtime-temperature gradient.

Again, these heating/cooling techniques may be used pre-bonding orin-situ. In connection with in-situ bonding techniques, the bondingapparatus described in, for example, U.S. patent application Ser. No.11/417,445, entitled HIGH TEMPERATURE ANODIC BONDING APPARATUS, theentire disclosure of which is hereby incorporated by reference, may beadapted for use in accordance with the present invention. Management ofthermal radiation loss in the bonding apparatus may be controlled, andthus exploited to achieve the time-temperature gradient, through theincorporation of infrared reflecting elements around the perimeter ofthe bonding apparatus to minimize radiation loss and maximize edgetemperature. Conversely, management of thermal radiation loss in thebonding apparatus may be controlled through the incorporation of cooledinfrared absorbers to maximize radiation loss and minimize edgetemperature. Many variations on the above themes may be used to achievethe desired time-temperature gradient.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. A method of forming a semiconductor-on-insulator (SOI) structure,comprising: subjecting an implantation surface of a donor semiconductorwafer to an ion implantation step to create a weakened slice incross-section defining an exfoliation layer of the donor semiconductorwafer, where the donor semiconductor wafer has a width, a depth and aheight, the width and depth defining X- and Y- axial directions, and theheight defining a longitudinal axis; and subjecting the donorsemiconductor wafer to a spatial variation step during the ionimplantation step, such that densities of nucleation sites resultingfrom the ion implantation step vary spatially across the weakened slicein at least one of the X- and Y- axial directions, wherein the ionimplantation step includes: (i) implanting a first species of ions in asubstantially uniform manner to create a weakened slice of substantiallyuniform distribution; and (ii) implanting a second species of ions in asubstantially non-uniform manner, such that the second species of ionscauses migration of atoms to the weakened slice resulting in thespatially varying densities of nucleation sites across the weakenedslice.
 2. The method of claim 1, wherein the first species of ions ishydrogen and the second species of ions is helium.
 3. The method ofclaim 1, wherein the step of implanting the second species of ions in asubstantially non-uniform manner includes implanting the second speciesof ions to varying depths spatially across the donor semiconductorwafer.
 4. The method of claim 1, wherein a maximum local density ofnucleation sites exists in a first region of the weakened slice of about5×10⁵ sites/cm² and a minimum density of nucleation sites exists in asecond region of the weakened slice of about 5×10⁴ sites/cm², where thesecond region is spaced away from the first region in at least one ofthe X- and Y- axial directions.
 5. The method of claim 1, wherein amaximum density of nucleation sites in a first region of the weakenedslice is at least 10 times of a minimum density of nucleation sites in asecond region of the weakened slice.
 6. The method of claim 1, furthercomprising elevating the donor semiconductor wafer to a temperaturesufficient to initiate separation at the weakened slice from a point,edge, and/or region of highest density of nucleation sites.
 7. Themethod of claim 6, further comprising elevating the donor semiconductorwafer to further temperatures sufficient to continue separationsubstantially along the weakened slice directionally as a function ofthe varying densities of nucleation sites, from highest density tolowest density.
 8. The method of claim 7, wherein a time-temperatureprofile of the elevating temperatures is on the order of seconds, suchthat a propagation of the separation along the weakened slice fromhighest density to lowest density occurs over at least one second. 9.The method of claim 1, wherein the spatial variation step includesspatially varying a dose of implanted ions in at least one of the X- andY- axial directions.
 10. The method of claim 9, wherein the spatialvariation step includes spatially varying the dose of implanted ionssuch that a substantially high dose exists along an initial edge, aninitial point, or an initial region of the weakened slice of the donorsemiconductor wafer and comparatively lower dosages exist atsuccessively further distances from the initial edge, the initial point,or the initial region in at least one of the X- and Y- axial directions.11. The method of claim 10, wherein the lowest dose is from 70% to 90%of the substantially high dose, in certain embodiments at least 80%. 12.The method of claim 10, wherein the substantially high dose exists at aninitial point or region along one or more edges of the weakened sliceand comparatively lower dosages exist at successively further distancesfrom the initial point or region in both the X- and Y- axial directions.13. The method of claim 9, wherein: the donor semiconductor wafer isrectangular; and the spatial variation step includes spatially varyingthe dose of implanted ions such that a substantially high dose exists ateach of at least two edges of the weakened slice of the donorsemiconductor wafer and comparatively lower dosages exist atsuccessively further distances from the at least two edges toward acenter of the weakened slice.
 14. The method of claim 13, wherein thespatial variation step includes spatially varying the dose of implantedions such that a substantially high dose exists at all edges of theweakened slice and comparatively lower dosages exist at successivelyfurther distances toward the center of the weakened slice.